Method and means for providing enhanced main beam nulling in antijamming antenna

ABSTRACT

In a microwave communications link, an antijamming antenna system with enhanced main beam nulling is realized by providing gain in the difference port output of a monopulse antenna compared to the sum port. As a consequence interference signals are nulled out at much closer angles to the desired signal than is the case when the gain is the same in both channels. The gain is introduced when a dominant jammer is detected. Detection of a dominant jammer is identified for the conditions in which the monopulse sum channel power level exceeds the sum of the difference channel power levels and the system processor output signal/noise ratio is unsatisfactory.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

BACKGROUND OF THE INVENTION

This invention relates to antijamming antenna techniques for reducingthe vulnerability of line of sight and troposcatter microwavecommunications links to jamming.

Specifically, the invention comprehends an antenna with an improved gaincontrol circuit for enhancing the null performance of the antennaagainst a jammer close to antenna boresight.

It has been demonstrated that in existing Defense communications systemsthere exists the need to protect all directions of arrival of a desiredsignal against jamming; jamming effectiveness is greatest in the mainbeam and near-in sidelobe regions of the antenna. Computedjamming/signal ratios for various simulated encounters has showed thatan integrated adaptive main beam-sidelobe canceller antenna is requiredto significantly reduce communication outages due to the various jammingencounters.

Antijamming antenna techniques have been developed which reducevulnerability of line of sight and troposcatter microwave communicationslinks to jamming. The techniques are incorporated in an antenna systemthat is configured to adaptively place a null in the direction of ajammer with an arrival angle close to the direction of arrival of adecreased signal (within the main beam) as well as a jammer arriving inthe sidelobe region of the main antenna. Spatial discrmination preventsnulling of the desired signal in the absence of jamming an improvedsignal/jamming ratio for jamming levels comparable to the desired signallevel. The antenna system utilized includes an auxiliary antenna thatconsists of a dual plane, multimode, monopulse feed which replaces thestandard feed, a low gain slot array and an omnidirectional antenna.

One problem encountered in the system developed was that of theinability to effectively null jammers that are close to the direction ofarrival of the desired signal. In this regard it has been determined,and it is fundamental to the present invention that nulling jammersclose to the direction of arrival of the desired signal than isindicated by existing techniques can be achieved at the expense ofsignal/noise ratio. This represents a trade off that is beneficial forsome scenarios.

The method of the present invention of increasing null performance forjammers close to the desired signal DOA is to increase the gain of themonopulse antenna auxiliary difference beams. This can be accomplishedspatially or by electronic means. For a specific reflector antenna theaperture size and spatial gain available is already fixed and onlyelectronic gain is available. As indicated above, the penalty for usingelectronic gain is an increase in the System Noise floor. A particularcharacteristic of line of sight microwave communication links of thetype to which the invention pertains is that they are designated to havelarge fade margins and therefore the received signal at one of thediversity antennas is usually strong and well above the noise floor.Advantage of this can be taken by the AJ antenna by increasing theelectronic gain in the monopulse antenna auxiliary difference beamchannels in a controlled manner. This permits nulling a jammer closer toboresight of the antenna. The resulting increase in noise does notdegrade system performance in cases where the signal is well above noisefloor at one of the diversity antennas.

SUMMARY OF THE INVENTION

The invention comprises an antenna system with an improved gain controlcircuit for enhancing null; performance of the antenna against a jammerclose to antenna boresight. The antenna system is provided with anauxiliary monopulse system having sum azimuth difference and elevationdifference ports. An adaptive system senses the auxiliary port outputsin conjunction with the signal/noise ratio of the antenna processoroutput and increases the gain in the difference circuits in response tocertain conditions that indicate the presence of a dominant jammer. Adominant jammer is indicated when the power level in the sum portexceeds the power levels of both difference ports and the signal/noiseratio of the processor output is unsatisfactory.

It is a principal object of the invention to provide a new and improvedmethod for effecting main beam nulling in antijamming antennas.

It is another object of the invention to provide an improved method ofnulling out interference signals at angles close to boresight inantijamming antennas.

It is another object of the invention to provide, in an antijammingantenna, means for nulling jammers closer to the direction of arrival ofdesired signals than has heretofore been possible.

It is another object of the invention to provide an antijamming antennasystem having adaptive means for detecting the presence of and nullingout jammers close to boresight.

These together with other features and advantages of the invention willbecome more readily apparent from the following detailed descriptionwhen taken in conjunction with the illustrative embodiment in theaccompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the enhanced main beam processorconfiguration of the invention;

FIG. 2 is a performance curve of S/(J+N) vs angle of jammer offboresight with 0 electronic gain;

FIG. 3 is a performance curve of S/(J+N) vs angle of jammer offboresight with 3dB electronic gain;

FIG. 4 is a performance curve of S/(J+N) vs angle of jammer offboresight with 6dB electronic gain;

FIG. 5 is a performance curve of S/(J+N) vs angle of jammer offboresight with 9dB electronic gain; and

FIG. 6 is a simplified block diagram of the enhanced main beam nullconfiguration of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention provides enhanced main beam nulling in the antenna systemof a line of sight or troposcatter microwave communications link. Itcomprises a method and means for improving null performance for jammersclose to the desired signal direction of arrival.

A block diagram of an antenna configuration of the type to which theinvention may be applied is shown in FIG. 1. It comprises a monopulsesystem that includes complex feed 10, antenna combiner 13, auxiliaryreceivers 14, 15, main receiver 16, horn (or main) antenna 11,omnidirectional antenna 12, auxiliary receivers 26, 27, adaptive weights20-23 and processor 24. Variable gains 18, 19 in the azimuth andelevation difference channel of the monopulse system provide additionalgain in accordance with the principles of the invention.

In practicing the invention a controlled amount of gain is introducedinto the difference ports of the monpulse system by means of variablegains 18 and/or 19. As a result an interference signal can be nulledthat is much closer in angle to the desired signal than would be thecase when the gain is the same in both channels.

The basic principle of the invention is illustrated in FIGS. 2-5. Forthese cases a J/N of +60 dB and JIN of +20 dB were used. In developingthe curves of: FIGS. 2-5 the desired signal was centered on the peak ofthe beam while the jammer was moved in azimuth from the beam peak tofive degrees away from the peak. The S/J+N ratio is plotted with andwithout the AJ antenna processor.

FIG. 2 demonstrates the procedure with no electronic gain in the mainauxiliary path. FIG. 3 shows the effect of adding 3 dB of electronicgain in the A AZ auxiliary port. The adapted response in the vicinity ofpeak has sharpened considerably, that is the jammer can be closer to thedirection of arrival (DOA) of the signal and be nulled. The tesult ofincreasing the gain from 3 dB to 6 dB is shown in FIG. 4. Additionalsharpening of the adapted response is noted. Increasing the gain stillfurther to 9 dB (FIG. 5) produces little additional sharpening andbegins to affect the sdidelobe region. There are several problems inapplying this approach to the system. For example, if a fixed amount ofgain were inserted in the difference beam auxiliaries, the effectivenessof the spatial discriminant would be reduced and a loss in S/N wouldoccur for scenarios where the sharpended response is not required. Theseproblems can be alleviated by making use of the jammer detection andperformance monitoring ciruits shown in FIG. 6.

FIG. 6 is a simplified block diagram of the enhanced main beam nullconfiguration. In addition to the antenna elements common to those shownin FIG. 1 this configuration includes an angle discriminant 30, asignal/noise measurement device 32 and gain control logic 31. These areconventional radar system components and perform their requiredfunctions in the normal way.

The monopulse feed is an ideal direction of arrival indicator. That isthe jammer's arrival angle with respect to antenna boresight can bedetermined accurately for relatively small J/S ratios. A simple buteffective approach, to apply the enhanced nulling techniques adaptively,senses the power in the sum port (non-processed) and Az/EL differenceports. If the power in the sum port exceeds the power sensed in bothdifference ports, it indicates that a signal is in the angular conearound boresight when the normal processor is not effective. Thequestion is whether this is the desired signal alone or is a dominantjammer also present. This is determined by the performance monitoringcircuit. If satisfactory S/N is indicated for SIN measure 32, theprocessor 24 remains in its normal configuration. If S/N isunsatisfactory, it indicates a jammer is within the sector where normalprocessing is ineffective. These measurements are monitored by a gaincontrol logic circuit 31 which selects the additional gain for enhancedmainbeam null performance as may be required. The gain control logicalso provides stabilization control to prevent constant gain switchingfor pulsed or fading conditions.

While the invention has been described in one presently preferredembodiment it is understood that the words which have been used arewords of description rather than words of limitation and that changeswithin the purview of the appended claims may be made without departingfrom the scope and spirit of the invention in its broader aspects.

What is claimed is:
 1. In an antijamming antenna system having a mainantenna, an omnidirectional antenna, a monopulse antenna with azimuthand elevation difference beam ports each having gain and a sum beamport, and a processor receiving signals therefrom and providing aprocessed output, the method of enhancing main beam nulling comprisingthe step of increasing gain in said difference beam ports.
 2. In anantijamming antenna system having a main antenna, an omnidirectionalantenna, a monopulse antenna with azimuth and elevation difference beamports each having gain and a sum beam port, and a processor receivingsignals therefrom and providing a processed output, the method ofenhancing main beam nulling comprising the steps of detecting thepresence of a dominant jamming signal from the power levels in saidmonopulse antenna sum and difference beam ports and the signal/noiseratio of said processed output, and increasing gain in said differencebeam ports in response thereto.
 3. In an antijamming antenna systemhaving a main antenna, an omnidirectional antenna, a monopulse antennawith azimuth and elevation difference beam ports each having gain and asum beam port, and a processor receiving signals therefrom and providinga processed output, the method of enhancing main beam nulling defined inclaim 2 wherein the presence of a dominant jamming signal is detected bythe steps of sensing the power level in said monopulse antenna sum beamport, sensing the power levels in said monopulse antenna difference beamports, determining if the signal/noise ratio of said processed output issatisfactory or unsatisfactory, and identifying the presence of adominant jamming signal for the condition that the power in the sum beamport exceeds the power in both difference beam ports and thesignal/noise ratio of said processed output is unsatisfactory.
 4. In anantijamming antenna system having a main antenna, an omnidirectionalantenna, a monopulse antenna with azimuth and elevation difference beamports each having gain and a sum beam port, and a processor receivingsignal therefrom and providing a processed output, the method ofenhancing main beam nulling defined in claim 3 wherein gain in saiddifference beam ports is increased by an amount in a range between 3dBand 9dB.
 5. In a microwave communications link, an aritijamming antennasystem comprising a main antenna, an omnidirectional antenna, amonopulse antenna outputting azimuth and elevation difference signalsand sum signals, processor means receiving and processing the outputs ofsaid main antennas, said omnidirectional antenna and said monopulseantenna azimuth and elevation difference signals and sum signals, andproviding a processed output therefrom, jamming signal detecting meansfor detecting and outputting a signal in response to the presence of adominant jamming signal, said jamming signal detecting means detectingjamming signals from the power levels of said monopulse antenna azimuthand elevation difference signals and sum signals and the signal/noiseratio of said processed output, and gain control means for increasingthe gain of said moncpulse antenna azimuth and elevation differencesignals in response to the jamming signal detecting means output signal.6. In a microwave communications link, an antijamming antenna as definedin claim 5 wherein said gain control means increases the gain of saidmonopulse antenna azimuth and elevation difference signals in responseto the conditions in which the power level of said sum signal is greaterthan the total power of said azimuth and elevation difference signalsand the signal/noise ratio of said processed output is less than apreselected threshold.
 7. In a microwave communications link, anantijamming antenna as defined in claim 6 wherein said gain controlmeans increases the gain of said monopulse azimuth and elevationdifference signals by an amount in a range between 3dB and 9dB.